Microwave frequency-selective mode absorber



Sept. 25, 1956 s. D. ROBERTSON 2,764,743

MICROWAVE FREQUENCY-SELECTIVE MODE ABSORBER Filed Dec. 30, 1949 A TTENUAT/ON- DEC/EELS PER CM.

0 I I 1 J 3900 4000 4/00 4200 4000 4400 0 1 1 l I L FREQUENCY-MC 0900 4000 4/00 4200 4000 4400 FREQUENCY-MC INVENTOR s. 0. ROBERTSON AGENT United States Patent 9 MICROWAVE FREQUENCY-SELECTIVEMODE ABSORBER 1 Application December 30, 19.49, Serial No. 136,119

21 Claims. (Cl. 333-98) assignor to Bell This invention relates, to microwave filters, networks and the like.

An object of this invention is to simplify the construction of frequency-selective. filters and networks utilizing waveguide components, particularly in the range. of l to lOO-millimeter wavelength.

Another object of the invention. is to: incorporate frequency-selective. characteristics in hollow conductors and wave. guides,. without introducing impedance mismatches or secondary reflections along the direction of propagation, by lining the Walls of the conductors or wave guides. with. thin. strips of metallically loaded dielectric material of high efiective dielectric constant k.

A. featureof the invention is a microwave frequencyselectivecircuit comprising a hollow wave guide lined with energy-absorbing dielectricnsheets, havingmetallic flakes dispersed. therethrough, to thereby provide effective dielectric constants/c of the order of 1.00 to 10,000in a direction parallel to the electric field. .vector E. The loaded dielectric sheets are aquarter wavelength. in effectivethickness for selectivity. and have a metallic foil backing for reflecting waves back into. the sheet. The sheets are. so thin with respect to. the. crossrsection. of the wave guide asto present correspondingly small transition discontinuities to. the propagated waves.

Heretofore, various types of wave-guide filters and networks were. known which utilized sections of wave guide providedwith. tuning v stubs, screws, adjustable irises and. fixed projections connected to the walls thereof. As the wavelength, decreased toward. the millimeter range, such devices. became increasingly diflicult tomanufaeture, particularly in view of the. restricted. size of cross-section in the hollow guidesand the, severe limits of manufacturing tolerance-involved.

Moreover, wave-guide selective circuits. of the past have: suffered from. the disadvantage of not possessing a constant impedance throughout the frequency-selective range- The frequency-selective linings of the present invention remove this disadvantage in that their presence in the wave guide does not alter substantially the impedance exhibited at the input or output of the containing waveguide section per se. The transmission. of waves through these lined guides is. substantially free from the presence of secondary reflected waves propagating along the principal wave-guide axis.

Accordingly, in accordance with various embodiments of the present invention, longitudinal sheets of loaded dielectric material, )\/4' in effective thickness, are provided as a lining for the walls of the wave guides, to thereby provide various frequency-selective characteristics. The loaded dielectric may consist of thin superposed layers of rubber or the like, having fine carbon and microscopic sized aluminum flakes or the like dispersed uniformly therethrough to provide absorption effects and dielectric constants k, as high as 1500 when the electric field vector is parallel to the sheet. Sheets of this material need be only a few mils thickto equal M4 for waves in the centimeter range, in order to. provide frequency selectivity characteristics in the wave guide lined thereby.

Electromagnetic wave energy of a narrow range of frequencies transmitted through such. a lined wave guide is either absorbed directly in the liningmaterial or reflected by the metal foilbacking into the dielectric sheet for additional absorption- The principal dimension of the anisotropic dielectric determining the absorption frequency thereof is its thickness. However, it has been discovered experimentally that the width of the longitudinal strip also effects the frequency absorbed, so that it can also be utilized as a secondary parameter for frequency variation.

Because of its absorptive characteristic, such a strip lining-the side wall of a wave guide, will' absorb selectively energy at a particular frequency from a wide band; of input waves, providing thereby a single-frequency ora narrow-band elimination filter. Wide band filtering can be accomplished by using two or more strips in 21 Wave guide; wherein the additive combination of the individual characteristics of the separate elimination filters aforementioned produces the requisite Wider band over-all characteristic.

Referring to the figures of the drawing Fig. 1 shows, in cross-section a sheet of metallica'lly loaded dielectric material, which is frequency selective;

Fig. 2 shows a rectangular wave-guide filter lined therewith in accordance with the invention;

Fig. 2A. is an explanatory diagram for illustrating the wave propagation'in a lined, wave guide;

Fig. 3 shows attenuation-frequency characteristic curves for the filter of Fig. 2; p I

Fig. 4A shows a wide, band, elimination wave-guide filter in accordance with the invention;

Fig. 4B shows its attenuation. versus frequency curve;

Fig. 5 shows a modification of the Fig.2 type of waveguide filter;

Figs. 6A and 6B show modifications utilizing tapered lining sheets i-n a wave guide; and

Fig. 7 shows a lined cylindrical wave guide used as. a frequency filter for TEOI waves.

Fig. 1 shows a sheet of material known. in the, art as Harp material (i. e., Halperns'Anti Radar Paint) or a similar form of loaded dielectric, having an effective thickness of one-quarter wavelength, and mountedon a reflecting 'surface which comprises a backing of metal foil 2. The loaded dielectric 1 consists of a rubber matrix 3. having small aluminum flakes 4 or the like dispersed there.- through rather uniformly with their surfaces. and, major axes parallel to the metal foil, 2. With aluminum loaded dielectrics, as in Fig. 1, effective. dielectric constants k as high as 1500' for IO-centimeter waves, have been attained. In addition, the sheet 1 contains some lossy, absorbing material, such as. fine carbon particles. or the like admixed therethrough.

The sheet 1 may be. formed by spraying. or painting successive layers. of a liquid. rubber having admixed therein lossy carbon particles andv microscopically fine aluminum flakes for attaining high dielectric constants k as a forementioned. Each deposited: layer has a thickness less than the length of the flakes, whereby the latter orient themselves parallel to the backingsurface, 2. The first layer is deposited on the backing 2 and allowed to dry, whereupon successive layers. are similarly deposited until the proper thicknessis achieved. This may be determined experimentally by placing a horn transmitter and a. hornreceiver above the backing plate and measuring the received power as the successive layers are built up. A sheet effectively 4 in thickness, where 7\ is the wavelength of incident microwaves, may in this Patented Sept. 25, 1956 manner he built up, which will be critically selective and absorb the energy of the incident wavelength When the rubber hardens, it provides a supporting matrix for the metal flakes and provides electrical insulation therebetween.

The required electrical conductivity of the sheet 1 is given by:

where k is the effective dielectric constant of the loaded sheet parallel to the electric vector E and v the frequency. In practice, absorption coeflicients obtained are of the order to 95 to 98 per cent.

The dielectric thickness d required for maximum absorption in the sheet is:

The loaded dielectric sheet 1 is anisotropic due to the layering in its mode of manufacture and exhibits the high dielectric constant k aforementioned when the electric vector E of the propagating waves is parallel to backing surface 2.

Microwaves incident upon the top surface 5 are partially reflected and partially transmitted as shown in Fig. 1. The transmitted component undergoes loss in passing through the lossy dielectric 1 toward the metal foil 2, which then reflects it so that it arrives again at the surface with a reversal in phase and with added loss. If the lossy admixture in sheet 1 be properly proportioned, the transmitted component will just cancel the initially reflected component at surface 5. Energy absorption occurs throughout the sheet over a fairly broad angle due to the high index of refraction of the loaded dielectric 1. Since the path length M4 in the sheet 1 is an important factor in creating the proper boundary conditions for wave cancellation at the incident surface 5, and since the path length is nearly constant over a wide range of incident angles, absorption occurs over relatively wide angles of incidence.

Fig. 2 shows a rectangular wave guide 20 havingits side walls 21 lined with identical sheets 22, 22 of loaded dielectric of the type disclosed previously in Fig. 1. However, a single lining sheet 22 may be used if desired, although two sheets as shown provide a greater total absorption of energy. In either case, impedance matching is provided over a wide band of frequencies due to the non-existence of secondary waves propagating along the guide axis.

Dominant waves E transmitted lar wave guide 20 are absorbed at a particular frequency by strips 22, 22' and it is possible to control the maximum or peak frequency of such absorption by either varying the thickness or width of the sheet. In order to make the resonance as sharp as possible, it is necessary to use as wide a strip as will fit inside the wave guide and then the resonant frequency is determined by adjusting the thickness of the dielectric sheet.

A preferred physical picture of this phenomenon may be presented thus. The dominant waves E transmitted through the rectangular wave guide 20 may be considered as composed of plane waves, which are reflected back and forth zigzag between the side walls 21 of the guide 20, and strike the side walls at an angle 0 as illustrated in Fig. 2A. When the component plane waves strike a dielectric strip 22, they are either absorbed or reflected therein depending upon the frequency. If they are reflected in the dielectric strip 22, the dominant wave propagates past unattenuated and unchanged in any way. If they are absorbed, the dominant wave disappears or is diminished.

Fig. 3 shows the frequency characteristics of the frequency filters disclosed in Fig. 2. As is apparent from the individual curves, these filters are peaked, narrowband elimination filters and it has been experimentally determined that filtering therein is accomplished withthrough the rectanguout objectionable reflection effects. The rectangular strips 22, 22' were cut from a large sheet of loaded material originally designed with a thickness 'of M4 to resonate at 9.1 centimeters wavelength and were mounted in standard one inch by two inches rectangular wave guide. The curves A, B, C, show how the resonant frequency v varies with the strip with w and also indicate that the maximum loss becomes lower as the width is decreased. The attenuation or loss provided by a single strip is linear with length, and doubling of length doubles the attenuation in decibels. The length of strip utilized in connection with curves A, B, C was approximately five inches.

It is possible to produce a variety of attenuations versus frequency characteristics by combining strips of material having various widths and lengths. One may also utilize various shapes for the strips in order to obtain a variety of transmission characteristics in connection with concentric conductor or hollow wave-guide transmission.

Fig. 4A shows a wide-band elimination filter consisting of strips of two different widths w, w respectively mounted on opposite sides of the rectangular wave guide 40. It will be noted that the strips are arranged in pairs, so that the strips having the same width are opposite one another in the wave'gude.

Under these circumstances, the corresponding wideband elimination filter characteristic is as shown in Fig. 4B. The broad band results from the overlapping of resonant characteristics of the type shown in Fig. 3, further indicating inappreciable coupling or interaction between the strips. Should the corresponding strips on opposite sides of the rectangular wave guide 40 have different widths, then the resultant curve has still a band elimination characteristic, but is not compounded by the simple addition of the characteristics of the individual strips, suggesting the existence of a coupling or interaction between the strips.

Fig. 5 shows a folded U-shaped dielectric strip 51 lining a wave guide 50 along the top, bottom and side walls. In one case, the strip 51, which was five inches long, was folded into a U-shape to line an X or 3-centimeter band standard wave guide, and the filter displayed a resonance peak shifted below the frequency obtained with a single side strip per se.

Instead of combining strips of differing widths as in Fig. 4A, one might use a single strip 61 tapered as shown in Fig. 6A or a strip 62 having a stepped taper as in Fig. 6B, whereby various amplitude versus frequency characteristics of the broad band filter type are provided.

It should be understood that band-pass filters may be provided by combining a plurality of separate tapered strips. Each strip provides individually a band elimination effect, which attenuates frequencies outside the desired limits of the pass band. In a similar manner, highpass or low-pass filters can be constructed.

Since the loaded sheet 1 of Fig. l absorbs in a narrow range of frequencies and reflects per so at all other frequencies, it may be mounted on the interior surfaces of cylindrical wave guides capable of carrying higher order modes.

A TEol mode in such a cylindrical wave guide may be resolved into component plane waves, which strike the cylinder walls at an angle.

Fig. 7 shows a mode filter for transmitting exclusively a TEo1 mode in a cylindrical wave guide. It may be produced by coating the inside walls of the cylindrical guide 70 to an effective depth or thickness of M4 to provide a loaded dielectric lining or insert 71 in the interior of the guide. The symmetry of this arrangement provides a single frequency filter for the TEM mode, without concomitantly giving rise to spurious higher order modes, as might develop in an iris, stub or screw type filter construction. Such mode filters or suitable modifications thereof may be similarly provided in concentric Conductor wave transmission.

It should be understood that the loaded dielectric strips may be supported in: a'wave guide: or: in a. cavity resonator at or away from the wallsthereot' forvarying the attenuation of low and high. powerxwaves propagated therethrough. They also may be moved toand fro. in a wave guide in known manner atv a periodic. rate or under the control of modulating signals toeffect the modulationof microwaves transmitted through-such guide.

Also the use of the foregoing lined wave-guide sec?- tions for equalizing transmission through hollow pipe long lines, where for example, the thickness of the strip is M4 at a frequency above or below the transmitted band, and in microwave radio relaylink'circuits, would likewise fall within the scope of the present-invention:

It will be evident that by the proper proportioning and compounding of strips, the: transmission versus frequency characteristic curve can be made to correspond to almost any desired shape and such composite devices may therefore be used in transmission control of phase, amplitude and frequency. I i

Likewise, the strips may be shaped and fashioned to provide higher order mode filters foruse in wave guide and concentric conductor transmission and in cavity resonators in accordance with the teachings. of theinvention.

It will be evident that the techniques described here are also applicable to the construction. of frequency-selective circuits for physical wavesother thaneleetromagnetic waves. One might, for example, apply. a similar layer of elastic material of a thickness corresponding to M 4 for a sound wave to theside walls of an acoustical horn or other propagating means in ordert'o; produce frequency-selective characteristics. I r

What is claimed is:

l. A mode filter comprising a hollow cylindrical wave guide having a loaded, longitudinal dielectric sheet lining its interior wall, said sheet being effectively M 4 in thickness, where is a wavelength propagated through said guide, said sheet comprising a lossy dielectric matrix with fine metallic particles parallelly disposed therein, said sheet having a reflecting backing thereon in contact with the guide wall.

2. A frequency-selective circuit comprising a hollow rectangular wave guide having a side wall thereof lined with a loaded dielectric sheet effectively M4 in thickness, where is a wavelength propagated through said guide, said sheet comprising a lossy dielectric matrix having fine metallic flakes distributed therethrough uniformly and lossy material admixed therewith.

3. A filter for microwaves comprising a rectangular hollow wave guide having a pair of very thin lining sheets therein longitudinally disposed therein, each in contact with one of a pair of parallel longitudinal walls of said guide, each sheet being lossy and having metallic flakes dispersed therethrough to provide an effective dielectric constant greater than 100 in a predetermined direction, each sheet being a quarter-wavelength in effective thickness and having a metallic foil backing, said lining sheets each being a few mils thick at centimeter wavelengths and thereby providing negligible discontinuities to the microwaves propagated through said guide.

4. A microwave filter comprising a short section of hollow wave guide having a critical frequency cut-off characteristic, an absorptive lining therein fixed to a longitudinal wall thereof comprising a loaded dielectric sheet having metallic flakes dispersed therethrough to provide an effectivedielectric constant k of the order of 100-10,000, said sheet having a thickness equal to characteristic impedance. of the" lined? section of wave guide thfiJClQSSfi-SGCIlGIlfiL dimensions; of thezholl'ow interior of said" guide being unappreciably altered by the-presence of saidzsh'eet. i

5:. A microwave filtercomprising a shortsecti'on of hollow rectangulan wave guide having aicritical'frequency cut-off characteristic, a longitudinal thin. sheet of: lossy dielectric having a. metallic reflecting foil. in contact with a wall of said guide, said sheet being loaded withmetallic' flakes oriented: parallel to: said. guide wall and having a dielectric constant k in the range of 1000; the thickness ofsaidsheetbeing equal toi 1 where A is the guide wavelength and the: characteristic impedance of said guide being suhstantially'unaltered by the; presence: of: said sheet, and: the hollow crossasection of said guide being unappreciably filled by said sheet. 6. A filter intaccordancewith claim- 5,. wherein said sheet comprises a;- series. of layers, eachcontaininga rubber matrix having, glossyparticles, and; said flakes being insulated from: each other by: saidmatrix,

7. The structure of claim 6, wherein the; flakes are aluminum oriented with their surfaces and. major axes parallelto. saith reflecting foil.

8-; The structure of claim 6,.wherein the sheet is. tapered alongits length toprovide. a bands filten characteristica 9; The structure of claim 5, wherein. the length; and width of said-lining; provides: a sharp: resonance absorption. v

10. The structure of claim 5, wherein a longitudinal sheet of equalzwidth is: mounted on the oppositezsid'e wall.

11. The structure of claim 5, wherein the lining-is U-shaped in cross-section and is in contact with the top, bottom and side walls of said guide.

12. A frequency-selective device comprising a hollow wave-guide section having open ends and being excited in a dominant mode, a loaded sheet of a few mils thickness lining a wall of said guide and having an efiective dielectric constant k greater than 100, said sheet comprising layers formed from a lossy dielectric matrix of low dielectric constant having metallic flakes therein oriented along parallel directions, said flakes being insulated from each other by said matrix, the length and width of said sheet determining the frequency characteristic of said device and the thickness of said sheet being where 7\ is the wavelength propagated through said wave guide between its ends.

13. The structure of claim 12, wherein the loss provided by said sheet-is linear with length, and a plurality of sheets collinearly spaced apart on one wall are contained in said wave guide to compound their frequency characteristics.

14. A filter for centimeter waves comprising a hollow conductor having open ends for incident electromagnetic waves, an absorber sheet having an effective high dielectric constant k 100, lining the wall of said conductor, said absorber being uniformly loaded with metallic particles oriented in one direction and insulated from each other, the thickness of said sheet being effectively one-quarter wavelength at the operating frequency, whereby the characteristic impedance of said conductor remains substantially unaltered throughout between its open ends.

15. A filter for microwaves comprising a section of hollow rectangular wave guide carrying waves in the dominant mode, a pair of thin, loaded dielectric sheets lining the side walls of said guide and spaced at least a half wave length apart, the characteristic impedance of said wave guide per se being substantially equal to the characteristic impedance of the lined section of wave guide, each sheet comprising a lossy dielectric matrix having metallic flakes dispersed therethrough to provide an effective dielectric constant 'k 100 and having a thickness effectively equal to one-quarter wavelength at the operating frequency to provide a phase reversal in incident wave components to provide frequency selective characteristics in said guide section without altering appreciably the internal cross-sectional dimensions of said section.

16. Frequency selective microwave apparatus comprising a conductive-walled hollow pipe wave guide adapted to propagate electromagnetic waves of predetermined field configuration and frequency, at least a portion of the conductive wall having a dissipative internal coating of vdielectric material that exhibits to said waves a dielectric constant of at least several hundred and an effective electrical thickness of the order of a quarter wavelength.

17. The apparatus of claim 16, wherein the dielectric material contains dissipative material dispersed throughout.

18; The apparatus of claim 16, wherein the dielectric materialis artificial, anisotropic, and contains metallic flakes in a predetermined orientation.

19. Frequency selective microwave apparatus comprising a hollow section of wave guide of rectangular cross-section adapted to propagate electromagnetic waves of predetermined field configuration and frequency, and a-film of lossy anisotropic dielectric material mounted therein parallel to the narrower side walls thereof and having a thickness equal effectively to a quarter wavelength at the operating frequency, the frequency selectivity of said film being controlled by its thickness and width.

20. The apparatus of claim 19, wherein the film thickness where k is the effective dielectric constant 100, and 71 is the wavelengthin the guide, said waves being selectively transmitted through said section substantially free from secondary reflected Waves arising from characteristic impedance mismatching.

21. A microwave filter comprising a short section of hollow wave guide supporting a dominant mode, an absorptive film having frequency selective characteristics fixed to a longitudinal wall thereof, said film comprising a dielectric sheet whose thickness is equal effectively to a quarter Wavelength at the operating frequency and having metallic particles dispersed therein to provide an effective dielectric constant k and having a width dimensioned to determine the absorbed frequency, the cross-section and characteristic impedance of said guide being substantially unaffected by said film to preclude generating secondary reflected waves.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,123 King Apr. 16, 1940 2,407,911 Tonks et a1 Sept. 17, 1946 2,423,396 Linder July 1, 1947 2,433,368 Johnson et a1 Dec. 30, 1947 2,449,182 Sontheimer Sept. 14, 1948 2,461,005 SOuthWOl'th Feb. 8, 1949 2,474,384 Sunstein June 28, 1949 2,656,518 Good Oct. 20, 1953 FOREIGN PATENTS 22,711 Australia May 21, 1935 802,728 France June 13, 1936 585,460 Great Britain Feb. 7, 1947 

